Can Viagra cause blood clots?

Erectile dysfunction can have a serious impact on a man’s confidence, relationships, and overall quality of life. However, many men hesitate to start treatment with Viagra (sildenafil) because of worries about side effects. One of the most common questions asked is: Does Viagra increase the risk of blood clots?
Blood clotting is a natural and vital process that stops bleeding after an injury. But when clots form inside the blood vessels without cause, they can block circulation and potentially trigger serious health problems such as heart attacks, strokes, or deep vein thrombosis (DVT). This is why concerns about Viagra and blood clots are often raised.
This blog explores the concerns around Viagra and blood clots, explaining how Viagra works, the risks involved, and what every man should know before taking it.

Key takeaways

• Viagra does not directly cause blood clots but may affect blood flow in ways that require caution for some men [6].
• Erectile dysfunction is often linked to cardiovascular health, which can increase the risk of clot formation [1], [7], [12].
• Viagra works by improving blood flow via PDE5 inhibition, not by thickening blood or causing clots [2].
• Men with certain cardiovascular conditions or on blood thinners should consult healthcare providers before using Viagra [4].
• Understanding medication interactions and individual health risks is key to safe and effective use of Viagra [3].

Understanding erectile dysfunction and cardiovascular health

Erectile dysfunction (ED) is commonly linked to cardiovascular issues, as both conditions involve blood vessel health and circulation problems. ED can be an early warning sign of underlying heart disease or blood flow issues that may also increase the risk of blood clots [7].

Research shows that men with ED are more likely to have risk factors like high blood pressure, diabetes, and high cholesterol, all of which can contribute to the formation of blood clots [7].

In fact, ED is sometimes described as a “canary in the coal mine” for cardiovascular health, making it essential to look at the bigger picture if you’re experiencing symptoms [11], [12].

Viagra-associated deaths and the mystery of blood clots

Since Viagra first hit the market, there have been occasional reports of heart attacks and strokes—some fatal—in men taking the drug. This puzzled researchers and the public alike, especially since Viagra was initially developed as a treatment for angina (chest pain), thanks to its blood vessel-widening properties [6].

Ironically, it was only after a serendipitous discovery of its side effect—erections—that Viagra became the world’s most famous little blue pill [6].

How Viagra works

Viagra contains the active ingredient, sildenafil and works by increasing levels of cGMP, a molecule that relaxes blood vessels and lets more blood flow into the penis [13]. Early on, scientists believed that cGMP might actually prevent blood clots by stopping platelets (the sticky cells that help blood clot) from clumping together [6]. The thinking was that more cGMP would result in fewer clots. Simple, right?

Platelets, cGMP, and a scientific plot twist

Let’s quickly recap: platelets are tiny, disc-shaped cells in your blood that rush to plug up wounds and stop bleeding. That’s great for a cut finger, but if platelets clump inside a blood vessel, it can lead to a dangerous clot (thrombosis), potentially causing a heart attack or stroke.

The twist came in 2003, when a research team led by Xiaoping Du at the University of Illinois at Chicago discovered that PKG, short for cGMP-dependent protein kinase, actually made platelets stickier under certain conditions [14].

When they tested Viagra on mouse and human platelets, they saw the same sticky response—especially in environments that mimicked damaged blood vessels (like those found in people with atherosclerosis, or hardening of the arteries) [14].

Injured vessels, Viagra and clots

So, alone, Viagra didn’t make platelets clump. But in the presence of “injured” vessels, it could make things stickier—even at doses lower than those used for erectile dysfunction.

As Du put it, “Viagra, by itself, probably is not sufficient to cause a heart attack in healthy people, but our research suggests that it may present a risk for patients with pre-existing conditions such as atherosclerosis.”

Does Viagra cause blood clots in healthy men?

In healthy vessels, Viagra had no effect on platelets. But for men with already narrowed or injured arteries, this clumping could, in theory, tip the balance toward a clot.

The good news: research shows that Viagra does not cause platelets to clump in healthy blood vessels. So, for men without significant cardiovascular risk factors, taking Viagra is not linked to an increased risk of blood clots [6].

The risk appears to rise only in individuals with pre-existing vessel damage or other risk factors for clotting.

New research: a tale of two sides

Since 2003, studies have painted a more nuanced picture of the situation. Some research suggests that PDE5 inhibitors (the class including Viagra) may actually reduce thrombosis risk in specific settings, such as after stroke, by inhibiting platelet aggregation through the cAMP/cGMP/NO pathway [15].

Other studies show that sildenafil can block platelet activation, mainly when triggered by certain clotting factors [16], [17]. There’s even evidence that, when given intravenously, Viagra can decrease platelet clumping [18].

Can Viagra cause clots?

So, does Viagra cause clots? The honest answer is: it’s complicated. Science is constantly evolving, and while most evidence suggests Viagra is safe for the vast majority, men with severe cardiovascular disease or damaged arteries should be extra cautious and always speak to a healthcare professional first.

The safety of Viagra

Viagra (sildenafil) works by blocking an enzyme called phosphodiesterase type 5 (PDE5), which helps relax blood vessels and increase blood flow to the penis during sexual arousal [2]. This mechanism improves erectile function but does not directly affect blood clotting or cause blood to thicken.

Multiple randomised controlled trials and meta-analyses have confirmed the safety of Viagra in men with and without cardiovascular disease, showing no significant increase in the risk of blood clots when used as prescribed [8], [9], [10].

Risks and precautions with Viagra

Men taking nitrates or certain blood pressure medications should not use Viagra due to the risk of dangerously low blood pressure. Additionally, those with a history of stroke, heart attack, or clotting disorders should consult their healthcare provider to assess risks and benefits [3].

Medication interactions and safe use

Understanding how Viagra interacts with other medications, especially blood thinners and cardiovascular drugs, is crucial [4].

Always disclose your complete medical history and current medications to your healthcare provider before starting Viagra to ensure safe use.

Studies have shown that, with proper screening, Viagra can be safely used by most men, including those on anticoagulants, provided there is appropriate medical oversight [4].

Who should not take Viagra?

Viagra is generally safe and well-tolerated, but certain groups should avoid it or use it only with medical supervision [2], [3] :

• Anyone taking nitrates for chest pain or heart conditions—combining these with Viagra can cause a life-threatening drop in blood pressure.
• People with very low blood pressure or those who are dehydrated.
• Anyone with allergies to sildenafil or its ingredients.
• Men with rare lung conditions, certain heart or blood flow problems, blood disorders like sickle cell anaemia, or a history of serious eye disease.
• Those with multiple system atrophy (a nervous system disorder) are at risk of dangerously low blood pressure.

Always check with your pharmacist or GP if you’re unsure. You can also contact us and speak to one of our prescribers at courierpharmacy.co.uk.

Conclusion

Viagra or sildenafil itself does not cause blood clots, but men with cardiovascular issues or clotting risks should use it cautiously and under medical supervision.

The research is reassuring for most users, but if you have complex health conditions or take multiple medications, personalised advice is essential.

Erectile dysfunction can be a sign of broader health concerns, so addressing these with a healthcare professional is key for safe and effective treatment.

FAQs

• Can Viagra cause blood clots? No, Viagra does not cause blood clots directly, but it may affect blood flow in men with pre-existing conditions [6].
• Is it safe to take Viagra with blood thinners? Only under medical supervision, as interactions can affect blood pressure and clotting [4].
• Does Viagra affect blood pressure? Yes, it can cause mild decreases in blood pressure, which is why caution is necessary when using certain medications [3].
• Who should avoid Viagra? Men taking nitrates, those with a recent heart attack or stroke, or those with certain cardiovascular conditions should avoid Viagra [2].
• Can Viagra cause a stroke? The risk is very low but may be increased in men with certain health conditions; always consult a healthcare provider [5].

Disclaimer: This article is for informational purposes only and does not replace professional medical advice.

References

1. NHS (2023) ‘Erection problems (erectile dysfunction)’. Available at: https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ (Accessed: 24 September 2025).
2. BNF (2025) ‘Sildenafil’. Available at: https://bnf.nice.org.uk/drugs/sildenafil/ (Accessed: 24 September 2025).
3. EMC (2025) ‘Sildenafil 100mg Tablets – Summary of Product Characteristics (SmPC)’. Available at: https://www.medicines.org.uk/emc/product/7124/smpc (Accessed: 24 September 2025).
4. Courier Pharmacy (2025) ‘Can you take Viagra with blood thinners?’ Available at: https://courierpharmacy.co.uk/can-you-take-viagra-with-blood-thinners-2/ (Accessed: 24 September 2025).
5. Medical Mojo (n.d.) ‘Can Viagra cause a stroke?’ Available at: https://medicalmojo.co.uk/can-viagra-cause-a-stroke/ (Accessed: 24 September 2025).
6. Medical Mojo (n.d.) ‘Can Viagra cause blood clots?’ Available at: https://medicalmojo.co.uk/can-viagra-cause-blood-clots/ (Accessed: 24 September 2025).
7. Orimoloye OA, Feldman DI, Blaha MJ. Erectile dysfunction is associated with cardiovascular disease, underscoring its clinical significance. Trends Cardiovasc Med. 2019 Nov;29(8):458-465.
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14. Li, Z., Xi, X., Gu, M., Feil, R., Richard, D.Y., Eigenthaler, M., Hofmann, F. and Du, X., 2003. A stimulatory role for cGMP-dependent protein kinase in platelet activation. Cell, 112(1), pp.77-86.
15. Gudmundsdóttir IJ, McRobbie SJ, Robinson SD, Newby DE, Megson IL (2005) Sildenafil potentiates nitric oxide-mediated inhibition of human platelet aggregation. Biochem Biophys Res Commun 337(1):382–385
16. Yang H-M, Jin S, Jang H, Kim J-Y, Lee J-E, Kim J, et al. (2019). Sildenafil reduces neointimal hyperplasia after angioplasty. It inhibits platelet aggregation via activation of cGMP-dependent protein kinase. Sci Rep 9(1):1–12
17. Berkels R, Klotz T, Sticht G, Englemann U, Klaus W (2001) Modulation of human platelet aggregation by the phosphodiesterase type 5 inhibitor sildenafil. J Cardiovasc Pharmacol 37(4):413–421
18. Lewis, G.D., Witzke, C., Colon-Hernandez, P., Guerrero, J.L., Bloch, K.D. and Semigran, M.J., 2006. Sildenafil improves coronary artery patency in a canine model of platelet-mediated cyclic coronary occlusion after thrombolysis. Journal of the American College of Cardiology, 47(7), pp.1471-1477.